1. Field of the Invention
The present invention relates to a method for measuring biomass. More particularly, it relates to a method for measuring on-line the quantities of microorganisms or the quantities of animal or plant cells in a bioreactor. Thus the present invention will play an important role in bioindustry and microbial industry relating to medicine, food, sewage disposal, etc.
2. Description of the Prior Art
The bioreactor to produce useful substances by the aid of microorganisms, animal or plant cells, etc. is different from the ordinary reactor containing catalyst in that the biomass in the reactor changes from time to time. Therefore, measuring the biomass is very important for the design and control of bioreactor. In the case of a bioreactor in which microorganisms are not immobilized on a support but suspended in the solution, it is possible to measure the quantities of the microorganisms by several methods. They include, for example, the measurement of turbidity with scattered light, the measurement of absorption spectrum by a double beam spectrometer, and the measurement of fine particles by a projecting light particle sensor.
These conventional measuring methods have some drawbacks. For example, the measurement of the concentration of microorganisms in activated sludge or industrial effluent, which is expressed in terms of MLSS (Mixed Liquor Suspended Solids), does not give an accurate number of microbial cells because sludge contains a variety of organic and inorganic matters in addition to microorganisms. It is also very difficult to separate microbial cells alone from sludge and measure their concentration. To approximate the value of MLSS to the concentration of microorganism, there has been proposed the measurement of MLVSS (Mixed Liquor Volatile Suspended Solids) in sludge. In actual, however, MLSS is still being used to denote the concentration of microorganisms in activated sludge because the measurement of MLVSS takes almost twice as much time as the measurement of MLSS.
For the measurement of MLSS, several methods have been developed; however, none of them are satisfactory. That is, the method that employs ultrasonic waves cannot be applied to a sample containing bubbles. The method that employs light is liable to errors when the measuring window is unclean and the sample solution is colored or bubbled, and it requires a complex measuring mechanism. The method that employs mechanical means or bubbles becomes paralyzed when foreign matters are encountered. For this reason, it is an actual practice to take a small sample and measure the dry weight of it by a complex procedure.
Other measuring methods are also used for plant cells and animal cells which have a larger volume than microorganisms and tend to form flocs. According to them, measurement is accomplished by weighing the dry weight, calculating the wet volume of cells, or counting the number of stained cells under a microscope. Whichever method may be used, it is necessary to take a sample of cells from the reactor or culture tank. This leads to the possibility of contamination with infectious microbes, and contamination often forces one to abandon an expensive culture medium. An additional disadvantage of these measuring methods is that the result of measurement such as the quantity of cells cannot be used for the on-line control of the culture equipment.
It is a recent common practice to increase the density of microorganisms in a bioreactor through their immobilization by chemical or physical means, thereby improving the efficiency of a bioreactor. A problem in this technical field is the fact that there is no method of measuring the quantity of immobilized microorganisms in a bioreactor without destroying the system at all. Instead of the direct method which is not available at present, the indirect methods as mentioned above are used for the microorganisms suspended in water after sampling from a bioreactor. Alternatively, the microorganisms sampled from a bioreactor are dried and their dry weight is measured. As long as such indirect methods are used, it is impossible to control a bioreactor on real time basis according to the results of measurements. Therefore, there has been a demand for the development of a new method for measuring on-line the quantities of microorganisms without interrupting a bioreactor.